The invention concerns a hydrostatic drive for raising and lowering and holding loads, in particular for elevators, having a working cylinder which is connected to a pressure fluid source and which forms a pressure chamber acted upon by a pressure fluid, a lift piston sealingly guided in the working cylinder and a guide rod which is arranged in the working cylinder and which projects into the interior of the lift piston which sealingly embraces the guide rod, wherein the end of the lift piston which projects into the pressure chamber forms an annular face which is acted upon by pressure fluid.
Such a drive is known from Austrian patent specification No 385 018 which is intended for raising and lowering loads over great heights. In the case of that drive, the fact that the end of the lift piston which projects into the working cylinder slides sealingly on the guide rod is intended to improve the level of safeguard against bending or buckling. The large diameter of the massive lift piston, which is otherwise required because of the aspect of safeguarding against bending or buckling, is now replaced by the annular face which forms the working face of the lift piston. The lift piston which is hollow because of the annular face is not acted upon at the inside by the pressure fluid. In the known drive, in each lifting operation, the entire lifting force is applied solely by the pressure fluid which is in the pressure chamber of the working cylinder and which acts on the annular face and which is fed to the working cylinder under variable pressure. The entire lift force therefore acts here over the entire length of the lift piston. In order to ensure the safeguard against bending or buckling, the wall thickness of the lift piston must be of correspondingly large size. A disadvantage is that with changing length of the lift piston its working face area increases, the through-flow quantity increases and a larger pump is required.
U.S. Pat. No. 2,269,786 describes a hydrostatic drive in which provided in the working cylinder for the drive of the elevator cabin beside the pressure chamber surrounding the guide rod is a second pressure chamber which passes through the guide rod, in contrast to Austrian patent specification No 385 018 therefore the guide rod is not of a circular cross-section but an annular cross-section. The fact that a pressure chamber is disposed in the interior of the guide rod ensures a higher degree of safeguard against buckling or bending of the guide rod although the amount of material required for the guide rod is markedly reduced, which at the time involves a significant saving in weight. In hydraulic terms, disposed between the two pressure chambers is a pump which can be switched over in respect of its direction of rotation and which can pump hydraulic fluid between the two pressure chambers. One of the two pressure chambers of the drive is in communication with a second hydraulic drive carrying a weight which acts as a counterweight for the elevator cabin.
The object of the invention is to improve a known hydrostatic drive in regard to the safeguard against bending or buckling, so that even greater structural lengths are possible.